1. Technical Field
The present invention relates generally to imaging systems which capture images having portions of high contrast and which reduce the intensity of portions of the image that exceed a predetermined threshold and, in particular, to an imaging system which employs a micromirror array having addressable sections of varying reflectivity so that portions of the image irradiating the micromirror array above a predetermined intensity threshold are reflected at or below the predetermined threshold by varying the reflectivity of each particular section.
2. Discussion
Many varied applications take advantage of the technology available for capturing an image and analyzing or processing the captured image. The captured image may be used by an image processing system for analysis and/or decision making. Examples of the various uses for imaging and image processing systems include astronomy, medical technology, weapon systems, and many other applications. For example, in astronomy, astronomers often use imaging technology in order to investigate sun spots or search for new planets or stars. In the medical field, imaging devices and image processors prove useful for x-ray procedures and for more advanced physiological scanning procedures such as computer assisted tomography scanning (CATSCAN) and magnetic resonance imaging (MRI). In the weaponry field, imaging devices and image processors prove useful for defensive targeting and destruction of both ground-based and airborne vehicles and munitions.
In each of the applications discussed above, the image captured by the imaging device typically includes a background or field of relatively low intensity, possibly objects of relatively medium intensity, and objects of relatively high intensity, to create a high contrast image. Present imaging systems and image processors discriminate between these relative intensities, but not without suffering a sometimes substantial loss or distortion of the captured image. When imaging devices and image processors seek only to capture light of a particular intensity and wavelength, various filters may be employed to eliminate light outside of a predetermined intensity and frequency band. Such filtration may resultantly cause distortion that is often severe enough to distort the particular object of interest. Thus, filtering the entirety of the captured image often results in an unacceptable distortion or even information loss in the captured image.
In a particular application, image trackers are often used in conjunction with lasers or other weaponry to disable in-flight missiles. Conventional image trackers presently employ only non-self-referencing schemes for directing a laser beam to a desired target aimpoint. In practice, this means that the laser beam direction in space is inferred from the tracker line of sight as the tracker tracks the missile.
Trackers using imaging, non-self-referencing techniques typically utilize one or more imaging devices, such as electronic cameras, that first determine an approximate, or wide field of view (WFOV) position, and then an instantaneous, or narrow field of view (NFOV), position of a targeted object. A target coordinate system is then typically established by determining the centroid of the target image. After capturing the target image in the NFOV's track gate, the tracker, under servo-loop control, follows the target. In most instances, the tracker is physically mounted on gimbals in a beam pointer. Therefore, the pointer line-of-sight also tracks the target if the pointer and tracker are properly boresighted.
Although conventional imaging, non-self-referencing trackers often provide adequate target location functions, a number of limitations exist with such systems. For example, in medium wave forward looking infrared (FLIR) based trackers, the laser weapon used for target engagement often interferes with the tracker imaging system, as instantaneous non-specular return from the laser hit spot on the object often blinds the camera, or, at least causes the camera automatic gain control to reduce camera gain to accommodate the bright laser hit spot, thereby losing all target image information. Typically, the laser-reflected power is some 40 to 60 dB greater than the target thermal signature. Additionally, with regard to long wave FLIR based systems, bright thermal energy from heated warheads may also blind such systems, causing the systems to lose track of the targeted object.
Solutions to the above problems include programming the system to select a laser aim point outside of the narrow field of view (NFOV) or the use of short wave infrared (SWIR) track bands with active illumination, causing the laser return to be invisible to the NFOV SWIR camera. If the laser aim point is selected outside of the view of NFOV however, the laser beam pointing must be determined by feed forward estimation. Such an aim point selection is undesirable, as it eliminates missile nose-kill possibilities, and is subject to estimation noise as explained earlier. Alternatively, if a SWIR track band is used, the laser beam pointing must also be done via feed forward estimation. Such a scheme increases the susceptibility of the tracker to atmospheric disturbances.
Additionally, with non-self-referencing imaging trackers, the tracker line-of-sight must be accurately boresighted with the laser weapon line of sight. Due to the design of such systems, it has been found difficult to maintain an accurate bore sight under adverse environmental conditions.
Self-referencing trackers solve the above described limitations of the conventional imaging, non-self-referencing trackers by referencing the laser beam instantaneous position to the target image itself rather than to the tracker line-of-sight direction. Also, self-referencing trackers have lines of sight that need not be coaxial with the laser weapon, thereby subsequently minimizing the weight on the system gimbals and simplifying system transmit optics.
Further, systems such as disclosed in pending U.S. patent application Ser. No. 08/631,645, entitled "Laser Crossbody Tracking System and Method", U.S. patent application Ser. No. 08/763,635, filed Dec. 4, 1996, entitled "A Novel Tracking Means for Distant Ballistic Missile Tracers," and U.S. patent application Ser. No. 08/760,434, filed Dec. 14, 1996, entitled "Laser Crossbody and Feature Curvature Tracker" ("the LACROSST patent applications"), both incorporated herein by reference, provide non-imaging self-referencing trackers.
In view of the foregoing, there is a need for an imaging system that can discriminate objects of differing intensities in high contrast images. Further, there is a need for an imaging system which filters out radiation at a particular intensity while limiting distortion of the remaining image. Further yet, there is a need for an adaptive focal plane imaging system having addressable sections to vary the intensity of sections of the captured image.